A DNA library derived from the B chromosome of Podisma kanoi was obtained by chromosome microdissection. A total of 153 DNA clones were isolated from the microdissected DNA library. Twenty of them were sequenced. A comparison of B chromosome DNA sequences with sequences of other species from the DDBJ/GenBank/EMBL database ( http://www.ddbj.nig.ac.jp/ ) was performed. Different patterns of signals were observed after FISH with labeled cloned DNA fragments. FISH signals with cloned DNA fragments painted either whole Bs or their different regions. Some clones also gave signals in pericentromeric regions of A chromosomes. Other cloned DNA fragments gave only background-like signals on A and B chromosomes. Comparative FISH analysis of B chromosomes in Podisma kanoi and P. sapporensis with DNA probes derived from the Bs of these species revealed homologous DNA that was confined within pericentromeric and telometric regions of the B chromosome in P. kanoi. In contrast to the B chromosomes in P. sapporensis containing large regions enriched with rDNA, only a small cluster of rDNA was detected in one of the examined B chromosomes in P. kanoi. The data strongly suggest an independent origin of B chromosomes in two closely related Podisma species.
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